The use of the Earth's magnetic field by many species for navigation has been widely documented although the sensory mechanism for detection remains elusive. Recent evidence suggests that the lagena, the third vestibular otolith receptor found in many non-mammalian vertebrates, may provide the answer. In birds and fish, the lagena epithelium has been shown to have a high content of iron and zinc, elements that could form magnetite compounds (e.g., Fe3O4), while the other two vestibular otolith organs (utricle and saccule) do not. X-ray spectroscopy and magnetic force microscopy will be used to examine the otolith organ receptor for the presence of magnetic dipoles (Aim 1). If present in the lagena, the dipoles could serve as a biophysical transducer for orientation in a magnetic field. Differing receptor hair cell polarizations across the lagena surface could then provide a spatial map upon which lagena afferents signal directions of orientation relative to the Earth main field, essentially acting as a magnetic compass. Whether lagena afferents encode magnetic spatial information will be examined using extracellular neural recording techniques (Aim 2). Other lagena afferents may encode linear accelerations in a vertical head plane. Both the spatial tuning and dynamic properties to linear motion and magnetic field activation will be examined in lagena afferents to determine if multimodal place fields exist in the lagena (Aim 2). Behaviorally, lagenar nerve section has been shown to disrupt normal homing ability in birds (Harada, 2002). We hypothesize that the lagena is involved in providing information to construct a geomagnetic map by central neurons for use in spatial navigation. In the final Aim (Aim 3), ablation studies will examine the effects upon magnetic field detection using behavioral orientation paradigms. Whether and how integration of magnetic compass, vestibular, and visual signals contribute to navigation and spatial orientation forms an exciting new direction of research that the current proposed investigation represents an integral initial step.